QtObject should be changed to Qt3DSBehavior

The way current behavior API works is that it expects a pure QtObject QML object that has some pre-defined functions in it. It also expects an XML header (for the editor application properties).

The QtObject is a bit abstract, it would be best to have a Qt3DSBehavior QML object the user uses and that should have the expected callbacks as signals (update, initialize etc.) instead of magic function names (onUpdate, onInitialize etc.). Also this class should be documented as QML object in standard Qt manner.

The assumption for this task is that the signal change will not impact calling of the defined functions in synchronous manner.